Chemical Quantities, The Mole, and Molar Mass – Study Notes

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Chemical Quantities and Reactions

The Mole Concept

The mole is a fundamental unit in chemistry used to express amounts of a chemical substance. It allows chemists to count atoms, molecules, or ions by weighing them. The mole is abbreviated as mol and is analogous to other counting units like a dozen (12 items).

Definition: 1 mole contains exactly $6.02 \times 10^{23}$ particles (Avogadro’s number).
Examples:
- 1 mole of H2O molecules = $6.02 \times 10^{23}$ H2O molecules
- 1 mole of C atoms = $6.02 \times 10^{23}$ C atoms
- 2 moles of C atoms = $2 \times 6.02 \times 10^{23} = 1.20 \times 10^{24}$ C atoms
Conversion Factors:
- $1\ \text{mol} = 6.02 \times 10^{23}$ atoms (or molecules)
- Can be used as a conversion factor between moles and number of particles.

Box of 12 donuts illustrating a dozen as a counting unit

Sample Problem: How many molecules are in 3.0 moles of NaF?

Given: 3.0 mol NaF
Conversion: $3.0\ \text{mol} \times \frac{6.02 \times 10^{23}\ \text{molecules}}{1\ \text{mol}} = 1.8 \times 10^{24}$ molecules NaF

Sample Problem: Calculate the number of atoms in 7.0 moles of sulfur (S).

$7.0\ \text{mol} \times \frac{6.02 \times 10^{23}\ \text{atoms}}{1\ \text{mol}} = 4.2 \times 10^{24}$ atoms S

Sample Problem: How many moles are in $2.11 \times 10^{24}$ atoms of H2?

$2.11 \times 10^{24}\ \text{atoms} \times \frac{1\ \text{mol}}{6.02 \times 10^{23}\ \text{atoms}} = 3.50\ \text{mol}$

Sample Problem: You have a sample containing $1.204 \times 10^{24}$ atoms of silver (Ag). How many moles of silver do you have?

$1.204 \times 10^{24}\ \text{atoms} \times \frac{1\ \text{mol}}{6.02 \times 10^{23}\ \text{atoms}} = 2.00\ \text{mol}$

Chemical Formulas, Formula Mass, and Molar Mass

Chemical formulas represent the types and numbers of atoms in a compound. The formula mass is the sum of the atomic weights of all atoms in a compound, measured in atomic mass units (amu). The molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol).

Chemical Formula: Combination of element symbols and subscripts indicating the number of each atom (e.g., H2O, NaCl, Ca(OH)2).
Formula Mass: Sum of atomic weights of all atoms in the formula.
Molar Mass: Formula mass expressed in grams per mole (g/mol).

Example: Calculate the formula mass and molar mass of MgSO4.

MgSO4 contains 1 Mg, 1 S, and 4 O atoms.
Atomic weights: Mg = 24.31 amu, S = 32.06 amu, O = 16.00 amu
Formula mass: $1 \times 24.31 + 1 \times 32.06 + 4 \times 16.00 = 120.37$ amu
Molar mass: 120.37 g/mol

Periodic table excerpt showing atomic weight and molar mass of carbon

Example: Calculate the molar mass of H2O.

2 H atoms × 1.01 amu = 2.02 amu
1 O atom × 16.00 amu = 16.00 amu
Formula mass = 18.02 amu
Molar mass = 18.02 g/mol

Example: Calculate the molar mass of CaCl2.

1 Ca atom × 40.08 amu = 40.08 amu
2 Cl atoms × 35.45 amu = 70.90 amu
Formula mass = 110.98 amu
Molar mass = 110.98 g/mol

Example: Calculate the molar mass of NH4Cl.

1 N atom × 14.01 amu = 14.01 amu
4 H atoms × 1.01 amu = 4.04 amu
1 Cl atom × 35.45 amu = 35.45 amu
Molar mass = 53.50 g/mol

Writing Chemical Formulas for Molecular Compounds

To write the chemical formula for a molecular compound, use the prefixes in the compound’s name to determine the number of each type of atom.

Example: Dihydrogen sulfide
"Di-" means 2 hydrogens; "sulfide" means 1 sulfur
Formula: H2S

Screenshot showing the formula H2S for dihydrogen sulfide

Periodic Table Groups and Ionic Compounds

The group number of a metal in the periodic table can often be determined from the formula of its ionic compound. For example, if a metal forms an ion with a 3+ charge (X3+) and combines with three F- ions to form XF3, the metal is likely from Group 3A.

Example: The formula XF3 indicates X is X3+, so X is from Group 3A.

Lecture Schedule and Chapter Overview

The following table summarizes the lecture schedule for a General Chemistry course, covering all major topics relevant to chemical quantities, reactions, and foundational concepts in chemistry.

Week	Date	Chapter	Quiz Dates
1	T 1/13	Ch. 1. Chemistry in Our Lives
2	T 1/15	Ch. 1. Chemistry in Our Lives
3	T 1/20	Ch. 2. Chemistry and Measurements	Quiz 1 opens due R 1/29
4	T 1/22	Ch. 2. Chemistry and Measurements
5	T 1/27	Ch. 3. Matter and Energy	Quiz 2 opens due R 2/5
6	T 2/10	Ch. 4. Atoms and Elements	Quiz 3 opens due T 2/17
7	T 2/12	Ch. 5. Nuclear Chemistry
8	T 2/17	EXAM 1
9	T 2/19	Ch. 6. Ionic and Molecular Compounds	Quiz 4 opens due T 3/3
10	T 2/24	Ch. 7. Chemical Quantities and Reactions	Quiz 5 opens due T 3/10
11	T 3/3	Ch. 8. Gases	Quiz 6 opens due T 3/24
12	T 3/5	Ch. 9. Solutions
13	T 3/10	Spring Break (NO CLASS)
14	T 3/12	Spring Break (NO CLASS)
15	T 3/17	EXAM 2
16	T 3/19	Ch. 11. Intro to Organic Chemistry: Hydrocarbons	Quiz 7 opens due R 4/2
17	T 3/24	Ch. 11. Intro to Organic Chemistry: Hydrocarbons
18	T 4/2	Ch. 12. Alcohols, Thiols, Ethers, Aldehydes, and Ketones	Quiz 8 opens due R 4/9

Lecture schedule for General Chemistry course

Summary Table: Molar Mass Calculations

Compound	Molar Mass (g/mol)
NH4Cl	53.50
C2H5OH	46.08
Al2(CO3)3	233.99

Additional info: The images of the flower bouquet, animal, and colored wall are not relevant to the chemistry content and are not included. The donut box image is used to illustrate the concept of a counting unit (dozen) analogous to the mole.